Answer with Step-by-step explanation:
We are given that a triangle ABC

We prove that 
We have to find the missing step in given proof.
Proof:
1.Statement:
Reason: Given
2.Statement:
Reason:If two parallel lines are cut by a transversal line then, congruent angles are congruent.
3.Statement:
Reason: AA criterion for similarity
4.Statement:
Reason:Corresponding sides of similar triangles are proportional
5.
Reason: Segment addition property
6.Statement:
Reason:Substitution property of equality
7.Statement:
Reason:Division property
8.Statement:
Reason: Subtraction property of equality
Hence, proved.